Prior to the production of integrated circuits, the semiconductor wafer upon which they are based is cut from a large silicon crystal. The wafer, which usually has a circular cross-section, has an upper surface, a lower surface, and an edge about the periphery of the wafer. Each of the surfaces and the edge of the wafer are processed prior to the lithographic processes which build integrated circuits upon the silicon wafer.
To assist in the rotational alignment of the wafer, a flat section or notch is ground into the edge of the wafer before photolithographic processing. The wafer must be repeatedly positioned and repositioned with tremendous precision so that the photolithographic processes will be properly aligned upon the wafer's surface and, more particularly, will be aligned with respect to the crystalographic planes of the wafer. With reference to the flat or notch (hereinafter collectively referred to as a “notch”), the wafer may be easily aligned.
As mentioned, the roughly cut wafer must be ground into its desired shape. Grinding of the edge gives the wafer its circular cross-section, but leaves a relatively rough and uneven surface along the edge of the wafer. Grinding of the notch into the edge of the semiconductor wafer also leaves a rough and uneven surface within the notch. The rough surface left after grinding of the edge or notch can, therefore, cause a variety of problems in future process steps. First and foremost, the grinding operation may cause particles to become trapped within the microstructure of the wafer. During subsequent processing steps, the trapped particles might leach out of the wafer and contaminate the wafer surface or the delicate wafer producing components. Also, physical abuse during grinding may cause local dislocations upon the surface of the edge or notch. The dislocations can potentially propagate into the wafer during subsequent high temperature processing steps, thereby harming the wafer and the integrated circuit based thereupon. Finally, residual material from the roughly ground edge or notch may become free during future processing and interfere with the lithographic process. Rough surfaces within a notch are particularly problematic because automated wafer handling devices repeatedly engage the notch during circuit processing and tend to dislodge any uneven fragments of silicon along the surface of the notch.
In order to avoid the problems associated with roughly ground edges and notches, silicon wafers are routinely polished along their edges or notches. Polishing is most typically done with a buffing wheel supplied with an abrasive slurry which polishes and smoothes the edge and notch surfaces of the wafer. Polishing of the edge and notch tends to remove any of the dislocations within the edge surface of the wafer, as well as removing protruding portions of the edge.
The slurry used in polishing the edge of the wafer is typically recycled by a slurry recycling apparatus which collects the used slurry from the wafer, filters the used slurry, and returns the slurry to the buffing or polishing pad. Continuous recycling and filtration of the polishing slurry theoretically leads to an uninterrupted supply of slurry. In commercial use, however, problems with proper filtration of the slurry material have plagued the industry. Improper filtration results in damage to the precision flow meters and other instrumentation used in monitoring slurry flow to the edge-notch polishing pad. When flow meters or other equipment are damaged by improper filtration, wafer processing must be suspended while the instrumentation is repaired or replaced.
Existing slurry filtering apparatuses consist of a gravity fed screen filter leading into an agitated tank. The polishing pad is supplied slurry from the agitated tank. The existing process has been ineffective at the reliable removal of polishing pad material and silicon material removed from the edge of the wafer. As such, contaminants such as the polishing pad material and silicon material previously removed from the edge of the wafer are recirculated to the edge-notch polishing pad. There contaminants can then prevent effective polishing of the edge.
An edge-notch polishing slurry recycling apparatus and method are needed to remove polishing pad material and large silicon material from the polishing slurry so that flow meters and other instrumentation within the recycling loop are not damaged and so that undesired particulate materials do not reach the edge-notch polishing pad, potentially harming the wafer. A slurry recycling apparatus and method is further needed that provides for continuous monitoring and maintenance of the slurry recycle apparatus so that interruptions in wafer processing are minimized.